Combustion-products detectors generally, and smoke detectors specifically, on the market today are generally of two basic types. In the photosensor type, a light from a source such as a light-emitting diode (LED) is normally directed away from a light-responsive device. In the presence of smoke, the light is caused by the smoke particles to strike the light-responsive device, thereby creating an alarm condition. Another kind of smoke sensor employs an ionization chamber defined by a pair of electrodes having a voltage thereacross. When products of combustion enter the chamber, the voltage is altered, signifying a smoke or fire condition.
There are three types of fires recognized by various approval/testing agencies. A smoldering fire is one in which a great deal of smoke is created, but very little flame. Smoldering fires can occur in upholstered furniture, for example, as the result of a burning cigarette. A smoldering fire can and often will progress into a flaming fire but it is desirable that the occupants be alerted as quickly as possible to the existence of a smoldering fire. A second type of fire produces what is commonly called "gray" smoke in which the particle size is small. In the third type "black" smoke is produced, which has a particle size between gray and smoldering smoke. A photoelectric sensor has greater sensitivity, than an ion detector, to smoke from a smoldering fire while an ion detector has greater sensitivity, than a photoelectric sensor, to black smoke.
In order to provide a combustion-products detector sensitive to all three types, there have been efforts in the past to utilize both photosensor circuitry and ionization sensor circuitry in the same combustion-products detector. In one of these prior circuits, the output of either circuit would activate the alerting-signal generator. However, these prior art detectors were subject to being falsed by air currents in the installation site. Ionization sensors are sensitive to drafts or other such movements of air and were, therefore, susceptible to falsing.